The doors of modem commercial aircraft are often provided with vent-latch interlock assemblies. A typical vent-latch interlock assembly includes a small pressure vent door that is seated in an opening formed in the aircraft door. A drive linkage is connected between the pressure vent door and a latch assembly that controls the opening, closing, and locking of the aircraft door. The drive linkage is arranged so that it will not close the pressure vent door until the associated aircraft door is latched and locked. Consequently, in the event the aircraft door is not properly closed, the open pressure vent door prevents the aircraft from being pressurized and thus requires aircraft personnel to take note that there is a problem with the door that requires attention. Moreover, a typical drive linkage is arranged so that, before the latch assembly is actuated to open the aircraft door, the pressure vent door is initially opened. The pressure vent door and the drive linkage are further constructed so that, when the aircraft cabin pressure is greater than the ambient pressure, the pressure vent door will not open and the drive linkage will not move. Since the drive linkage is integrally connected to the latch assembly, this "lockout" of the drive linkage prevents the actuation of the latch assembly and, thus, the unlocking and opening of the aircraft door. Thus, a pressure vent door assembly is well suited for installation to an aircraft cabin door to prevent the door from being opened while the aircraft is in pressurized flight. The pressure vent door and the drive linkage are further configured so that, in the event the ambient pressure is substantially greater than the aircraft cabin pressure, the pressure vent door will open. This allows air to bleed into the aircraft to reduce the pressure differential between the inside of the aircraft and the ambient environment. The minimization of this pressure differential reduces the force imposed on the aircraft by the surrounding atmosphere.
While current vent-latch interlock assemblies have proved useful, they are not without limitations. Sometimes when an aircraft lands, the pressure vent door is covered with ice and sealed in place. Many vent-latch interlock assemblies do not include a mechanism for breaking the pressure vent door free of the ice. Consequently, when the latch assembly is actuated to unlock and open the door, the pressure vent door remains closed. Since the pressure vent door will not open, the drive linkage locks out movement of the latch assembly and prevents the opening of the aircraft door.
Furthermore, often a vent-latch interlock assembly is constructed so that it locks out the movement of the door latch assembly only if the aircraft cabin pressure is significantly greater than the ambient pressure. If the cabin pressure is only slightly greater than the ambient pressure, the vent-latch interlock assembly will still allow the latch assembly to be actuated so that the door can be opened. Sometimes, if the aircraft cabin remains pressurized after the aircraft has landed, there can be a small pressure differential between the inside and outside of the aircraft. If the vent-latch interlock assembly does not prevent the latch assembly from being actuated, when the aircraft door is unlocked, the slightly higher cabin pressure may three the aircraft door rapidly outward. This rapid movement of the door has the potential to injure ground personnel, such as ramp attendants, who are standing outside of the aircraft.